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Tony Burton, Wind Energy Consultant, Powys, UKNick Jenkins, Cardiff University, UKDavid Sharpe, Wind Energy Consultant, Essex, UKErvin Bossanyi, GL Garrad Hassan, Bristol, UK

The authoritative reference on wind energy, now fully revised and updated to include offshore wind turbines and offshore wind farm development

A decade on from its fi rst release, the Wind Energy Handbook, Second Edition, refl ects the advances in technology underpinning the continued expansion of the global wind power sector. Harnessing their collective industrial and academic expertise, the authors provide a comprehensive introduction to wind turbine design and wind farm planning for onshore and offshore wind-powered electricity generation.

An all-important new chapter on offshore wind power covers:• resource assessment and array losses, optimal machine size and offshore turbine reliability• wave loading on support structures, including wind and wave load combinations and descriptions of

applicable wave theories• the different types of support structure deployed to date, with emphasis on monopoles, including fatigue

analysis in the frequency domain• the assessment of environmental impacts and the design of the power collection and transmission cable

network

Other new coverage features:• turbulence models updated to refl ect the latest design standards, including an introduction to the Mann

turbulence model• extended treatment of horizontal axis wind turbine aerodynamics, now including a survey of wind turbine

aerofoils, dynamic stall and computational fl uid dynamics• developments in turbine design codes, comparison of options for variable speed operation, and in-depth

treatment of individual blade pitch control • techniques for extrapolating extreme loads from simulation results• an introduction to the NREL cost model • grid code requirements and the principles governing the connection of large wind farms to transmission

networks• four pages of full-colour pictures that illustrate blade manufacture, turbine construction and offshore support

structure installation

Firmly established as an essential reference, Wind Energy Handbook, Second Edition, will prove a real asset to engineers, turbine designers and wind energy consultants both in industry and research. Advanced engineering students and new entrants to the wind energy sector will also fi nd it an invaluable resource.

Wind Energy Handbook

Wind Energy

Handbook

SECOND EDITION

Wind Energy Handbook SECOND EDITION

SECOND EDITION

Tony Burton

Nick Jenkins

David Sharpe

Ervin Bossanyi

BurtonJenkinsSharpe

Bossanyi

P1: OTE/OTE/SPH P2: OTEJWST051-FM JWST051-Burton April 8, 2011 11:59 Printer Name: Yet to Come



Second Edition

Tony BurtonWind Energy Consultant, Powys, UK

Nick JenkinsCardiff University, UK

David SharpeWind Energy Consultant, Essex, UK

Ervin BossanyiGL Garrad Hassan, Bristol, UK

A John Wiley and Sons, Ltd., Publication


This edition first published 2011C© 2011, John Wiley & Sons, Ltd

First Edition published in 2001

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Library of Congress Cataloguing-in-Publication Data

Wind energy handbook / Tony Burton . . . [et al.]. – 2nd ed.p. cm.

Includes bibliographical references and index.ISBN 978-0-470-69975-1 (hardback)

1. Wind power–Handbooks, manuals, etc. I. Burton, Tony, 1947–TJ820.H35 2011621.31′2136–dc22

2010053397

A catalogue record for this book is available from the British Library.

Print ISBN: 978-0-470-69975-1E-PDF ISBN: 978-1-119-99272-1O-book ISBN: 978-1-119-99271-4E-Pub ISBN: 978-1-119-99392-6mobi ISBN: 978-1-119-99393-3

Typeset in 10/12pt Times by Aptara Inc., New Delhi, India.

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Contents

About the Authors xvii

Preface to Second Edition xix

Acknowledgements for First Edition xxi

Acknowledgements for Second Edition xxiii

List of Symbols xxv

Figures C1 and C2 – Co-ordinate Systems xxxv

1 Introduction 11.1 Historical development 11.2 Modern wind turbines 41.3 Scope of the book 6

References 7Further reading 8

2 The wind resource 92.1 The nature of the wind 92.2 Geographical variation in the wind resource 102.3 Long-term wind speed variations 112.4 Annual and seasonal variations 122.5 Synoptic and diurnal variations 142.6 Turbulence 14

2.6.1 The nature of turbulence 142.6.2 The boundary layer 162.6.3 Turbulence intensity 182.6.4 Turbulence spectra 202.6.5 Length scales and other parameters 222.6.6 Asymptotic limits 242.6.7 Cross-spectra and coherence functions 252.6.8 The Mann model of turbulence 28


vi CONTENTS

2.7 Gust wind speeds 282.8 Extreme wind speeds 29

2.8.1 Extreme winds in standards 302.9 Wind speed prediction and forecasting 32

2.9.1 Statistical methods 322.9.2 Meteorological methods 33

2.10 Turbulence in wakes and wind farms 332.11 Turbulence in complex terrain 36

References 36

3 Aerodynamics of horizontal axis wind turbines 393.1 Introduction 393.2 The actuator disc concept 40

3.2.1 Simple momentum theory 413.2.2 Power coefficient 423.2.3 The Lanchester-Betz limit 433.2.4 The thrust coefficient 43

3.3 Rotor disc theory 443.3.1 Wake rotation 443.3.2 Angular momentum theory 463.3.3 Maximum power 48

3.4 Vortex cylinder model of the actuator disc 493.4.1 Introduction 493.4.2 Vortex cylinder theory 503.4.3 Relationship between bound circulation and the induced velocity 513.4.4 Root vortex 513.4.5 Torque and power 533.4.6 Axial flow field 533.4.7 Tangential flow field 533.4.8 Axial thrust 553.4.9 Radial flow field 563.4.10 Conclusions 57

3.5 Rotor blade theory (blade-element/momentum theory) 573.5.1 Introduction 573.5.2 Blade element theory 573.5.3 The blade-element/momentum (BEM) theory 593.5.4 Determination of rotor torque and power 62

3.6 Breakdown of the momentum theory 643.6.1 Free-stream/wake mixing 643.6.2 Modification of rotor thrust caused by flow separation 643.6.3 Empirical determination of thrust coefficient 65

3.7 Blade geometry 663.7.1 Introduction 663.7.2 Optimal design for variable speed operation 663.7.3 A simple blade design 703.7.4 Effects of drag on optimal blade design 733.7.5 Optimal blade design for constant speed operation 74


CONTENTS vii

3.8 The effects of a discrete number of blades 753.8.1 Introduction 753.8.2 Tip-losses 753.8.3 Prandtl’s approximation for the tip-loss factor 813.8.4 Blade root losses 833.8.5 Effect of tip-loss on optimum blade design and power 853.8.6 Incorporation of tip-loss for non-optimal operation 883.8.7 Alternative explanation for tip-loss 89

3.9 Stall delay 923.10 Calculated results for an actual turbine 953.11 The performance curves 97

3.11.1 Introduction 973.11.2 The CP − λ performance curve 983.11.3 The effect of solidity on performance 983.11.4 The CQ − λ curve 1003.11.5 The CT − λ curve 101

3.12 Constant rotational speed operation 1013.12.1 Introduction 1013.12.2 The K P − 1/λ curve 1013.12.3 Stall regulation 1023.12.4 Effect of rotational speed change 1033.12.5 Effect of blade pitch angle change 105

3.13 Pitch regulation 1053.13.1 Introduction 1053.13.2 Pitching to stall 1063.13.3 Pitching to feather 106

3.14 Comparison of measured with theoretical performance 1073.15 Variable speed operation 1083.16 Estimation of energy capture 1093.17 Wind turbine aerofoil design 114

3.17.1 Introduction 1143.17.2 The NREL aerofoils 1143.17.3 The Risø aerofoils 1163.17.4 The Delft aerofoils 117References 119Websites 120Further reading 120

Appendix A3 lift and drag of aerofoils 120A3.1 Definition of drag 121A3.2 Drag coefficient 123A3.3 The boundary layer 124A3.4 Boundary layer separation 124A3.5 Laminar and turbulent boundary layers 125A3.6 Definition of lift and its relationship to circulation 127A3.7 The stalled aerofoil 130A3.8 The lift coefficient 131


viii CONTENTS

A3.9 Aerofoil drag characteristics 131A3.10 Cambered aerofoils 134

4 Further aerodynamic topics for wind turbines 1374.1 Introduction 1374.2 The aerodynamics of turbines in steady yaw 137

4.2.1 Momentum theory for a turbine rotor in steady yaw 1384.2.2 Glauert’s momentum theory for the yawed rotor 1404.2.3 Vortex cylinder model of the yawed actuator disc 1444.2.4 Flow expansion 1464.2.5 Related theories 1524.2.6 Wake rotation for a turbine rotor in steady yaw 1524.2.7 The blade element theory for a turbine rotor in steady yaw 1544.2.8 The blade element – momentum theory for a rotor in

steady yaw 1554.2.9 Calculated values of induced velocity 158

4.3 The method of acceleration potential 1634.3.1 Introduction 1634.3.2 The general pressure distribution theory of Kinner 1654.3.3 The axi-symmetric pressure distributions 1684.3.4 The anti-symmetric pressure distributions 1714.3.5 The Pitt and Peters model 1744.3.6 The general acceleration potential method 1754.3.7 Comparison of methods 175

4.4 Unsteady flow 1764.4.1 Introduction 1764.4.2 Adaptation of the acceleration potential method to

unsteady flow 1774.4.3 Unsteady yawing and tilting moments 180

4.5 Quasi-steady aerofoil aerodynamics 1834.5.1 Introduction 1834.5.2 Aerodynamic forces caused by aerofoil acceleration 1844.5.3 The effect of the wake on aerofoil aerodynamics in

unsteady flow 1854.6 Dynamic stall 1894.7 Computational fluid dynamics 190

References 191Further reading 192

5 Design loads for horizontal axis wind turbines 1935.1 National and international standards 193

5.1.1 Historical development 1935.1.2 IEC 61400–1 1935.1.3 GL rules 194

5.2 Basis for design loads 1945.2.1 Sources of loading 1945.2.2 Ultimate loads 195


CONTENTS ix

5.2.3 Fatigue loads 1955.2.4 Partial safety factors 1955.2.5 Functions of the control and safety systems 197

5.3 Turbulence and wakes 1975.4 Extreme loads 199

5.4.1 Operational load cases 1995.4.2 Non-operational load cases 2025.4.3 Blade/tower clearance 2045.4.4 Constrained stochastic simulation of wind gusts 204

5.5 Fatigue loading 2055.5.1 Synthesis of fatigue load spectrum 205

5.6 Stationary blade loading 2055.6.1 Lift and drag coefficients 2055.6.2 Critical configuration for different machine types 2065.6.3 Dynamic response 206

5.7 Blade loads during operation 2135.7.1 Deterministic and stochastic load components 2135.7.2 Deterministic aerodynamic loads 2135.7.3 Gravity loads 2225.7.4 Deterministic inertia loads 2225.7.5 Stochastic aerodynamic loads: analysis in the frequency domain 2255.7.6 Stochastic aerodynamic loads: analysis in the time domain 2355.7.7 Extreme loads 238

5.8 Blade dynamic response 2415.8.1 Modal analysis 2415.8.2 Mode shapes and frequencies 2445.8.3 Centrifugal stiffening 2455.8.4 Aerodynamic and structural damping 2475.8.5 Response to deterministic loads: step-by-step dynamic analysis 2495.8.6 Response to stochastic loads 2545.8.7 Response to simulated loads 2565.8.8 Teeter motion 2565.8.9 Tower coupling 2615.8.10 Aeroelastic stability 266

5.9 Blade fatigue stresses 2675.9.1 Methodology for blade fatigue design 2675.9.2 Combination of deterministic and stochastic components 2695.9.3 Fatigue prediction in the frequency domain 2695.9.4 Wind simulation 2715.9.5 Fatigue cycle counting 272

5.10 Hub and low speed shaft loading 2735.10.1 Introduction 2735.10.2 Deterministic aerodynamic loads 2745.10.3 Stochastic aerodynamic loads 2755.10.4 Gravity loading 276

5.11 Nacelle loading 2775.11.1 Loadings from rotor 2775.11.2 Cladding loads 278


x CONTENTS

5.12 Tower loading 2785.12.1 Extreme loads 2785.12.2 Dynamic response to extreme loads 2795.12.3 Operational loads due to steady wind (deterministic component) 2825.12.4 Operational loads due to turbulence (stochastic component) 2835.12.5 Dynamic response to operational loads 2855.12.6 Fatigue loads and stresses 287

5.13 Wind turbine dynamic analysis codes 2885.14 Extrapolation of extreme loads from simulations 294

5.14.1 Derivation of empirical cumulative distribution functionof global extremes 295

5.14.2 Fitting an extreme value distribution to the empirical distribution 2965.14.3 Comparison of extreme value distributions 3015.14.4 Combination of probability distributions 3025.14.5 Extrapolation 3035.14.6 Fitting probability distribution after aggregation 3035.14.7 Local extremes method 3045.14.8 Convergence requirements 305References 306

Appendix 5: dynamic response of stationary blade in turbulent wind 308A5.1 Introduction 308A5.2 Frequency response function 309

A5.2.1 Equation of motion 309A5.2.2 Frequency response function 309

A5.3 Resonant displacement response ignoring wind variations along theblade 310A5.3.1 Linearisation of wind loading 310A5.3.2 First mode displacement response 311A5.3.3 Background and resonant response 311

A5.4 Effect of across-wind turbulence distribution on resonantdisplacement response 313A5.4.1 Formula for normalised co-spectrum 314

A5.5 Resonant root bending moment 316A5.6 Root bending moment background response 318A5.7 Peak response 319A5.8 Bending moments at intermediate blade positions 322

A5.8.1 Background response 322A5.8.2 Resonant response 322References 323

6 Conceptual design of horizontal axis wind turbines 3256.1 Introduction 3256.2 Rotor diameter 325

6.2.1 Cost modelling 3266.2.2 Simplified cost model for machine size optimisation an

illustration 3266.2.3 The NREL cost model 329


CONTENTS xi

6.2.4 Machine size growth 3316.2.5 Gravity limitations 332

6.3 Machine rating 3326.3.1 Simplified cost model for optimising machine rating in

relation to diameter 3326.3.2 Relationship between optimum rated wind speed and

annual mean 3346.3.3 Specific power of production machines 335

6.4 Rotational speed 3366.4.1 Ideal relationship between rotational speed and solidity 3366.4.2 Influence of rotational speed on blade weight 3376.4.3 Optimum rotational speed 3386.4.4 Noise constraint on rotational speed 3386.4.5 Visual considerations 338

6.5 Number of blades 3386.5.1 Overview 3386.5.2 Ideal relationship between number of blades, rotational

speed and solidity 3396.5.3 Some performance and cost comparisons 3396.5.4 Effect of number of blades on loads 3436.5.5 Noise constraint on rotational speed 3456.5.6 Visual appearance 3456.5.7 Single-bladed turbines 345

6.6 Teetering 3466.6.1 Load relief benefits 3466.6.2 Limitation of large excursions 3476.6.3 Pitch-teeter coupling 3486.6.4 Teeter stability on stall-regulated machines 348

6.7 Power control 3496.7.1 Passive stall control 3496.7.2 Active pitch control 3496.7.3 Passive pitch control 3546.7.4 Active stall control 3546.7.5 Yaw control 355

6.8 Braking systems 3566.8.1 Independent braking systems: requirements of standards 3566.8.2 Aerodynamic brake options 3566.8.3 Mechanical brake options 3586.8.4 Parking versus idling 358

6.9 Fixed speed, two speed or variable speed 3586.9.1 Two speed operation 3596.9.2 Variable slip operation (see also Chapter 8, Section 8.3.8) 3606.9.3 Variable speed operation 3616.9.4 Other approaches to variable speed operation 363

6.10 Type of generator 3656.10.1 Historical attempts to use synchronous generators 3656.10.2 Direct drive generators 367


xii CONTENTS

6.10.3 Evolution of generator systems 3686.11 Drive train mounting arrangement options 369

6.11.1 Low speed shaft mounting 3696.11.2 High speed shaft and generator mounting 372

6.12 Drive train compliance 3736.13 Rotor position with respect to tower 375

6.13.1 Upwind configuration 3756.13.2 Downwind configuration 376

6.14 Tower stiffness 3766.14.1 Stochastic thrust loading at blade passing frequency 3766.14.2 Tower top moment fluctuations due to blade pitch errors 3786.14.3 Tower top moment fluctuations due to rotor mass imbalance 3786.14.4 Tower stiffness categories 379

6.15 Personnel safety and access issues 379References 381

7 Component design 3837.1 Blades 383

7.1.1 Introduction 3837.1.2 Aerodynamic design 3847.1.3 Practical modifications to optimum design 3847.1.4 Form of blade structure 3857.1.5 Blade materials and properties 3867.1.6 Properties of glass/polyester and glass/epoxy composites 3907.1.7 Properties of wood laminates 3957.1.8 Blade loading overview 3987.1.9 Blade resonance 4097.1.10 Design against buckling 4147.1.11 Blade root fixings 418

7.2 Pitch bearings 4197.3 Rotor hub 4227.4 Gearbox 425

7.4.1 Introduction 4257.4.2 Variable loading during operation 4257.4.3 Drive train dynamics 4277.4.4 Braking loads 4277.4.5 Effect of variable loading on fatigue design of gear teeth 4297.4.6 Effect of variable loading on fatigue design of bearings

and shafts 4327.4.7 Gear arrangements 4337.4.8 Gearbox noise 4357.4.9 Integrated gearboxes 4367.4.10 Lubrication and cooling 4367.4.11 Gearbox efficiency 437

7.5 Generator 4377.5.1 Fixed-speed induction generators 4377.5.2 Variable slip induction generators 439


CONTENTS xiii

7.5.3 Variable speed operation 4407.5.4 Variable speed operation using a Doubly Fed Induction

Generator (DFIG) 4427.5.5 Variable speed operation using a Full Power Converter (FPG) 445

7.6 Mechanical brake 4467.6.1 Brake duty 4467.6.2 Factors governing brake design 4477.6.3 Calculation of brake disc temperature rise 4487.6.4 High speed shaft brake design 4507.6.5 Two level braking 4527.6.6 Low speed shaft brake design 453

7.7 Nacelle bedplate 4537.8 Yaw drive 4537.9 Tower 456

7.9.1 Introduction 4567.9.2 Constraints on first mode natural frequency 4567.9.3 Steel tubular towers 4577.9.4 Steel lattice towers 466

7.10 Foundations 4677.10.1 Slab foundations 4677.10.2 Multi-pile foundations 4687.10.3 Concrete monopile foundations 4687.10.4 Foundations for steel lattice towers 4697.10.5 Foundation rotational stiffness 469References 471

8 The controller 4758.1 Functions of the wind turbine controller 476

8.1.1 Supervisory control 4768.1.2 Closed loop control 4778.1.3 The safety system 477

8.2 Closed loop control: issues and objectives 4788.2.1 Pitch control (See also Chapter 3, Section 3.13 and

Chapter 6, Section 6.7.2) 4788.2.2 Stall control 4808.2.3 Generator torque control (see also Chapter 6, Section 6.9

and Chapter 7, Section 7.5) 4808.2.4 Yaw control 4818.2.5 Influence of the controller on loads 4818.2.6 Defining controller objectives 4828.2.7 PI and PID controllers 483

8.3 Closed loop control: general techniques 4848.3.1 Control of fixed speed, pitch regulated turbines 4848.3.2 Control of variable speed pitch regulated turbines 4858.3.3 Pitch control for variable speed turbines 4888.3.4 Switching between torque and pitch control 4888.3.5 Control of tower vibration 490


xiv CONTENTS

8.3.6 Control of drive train torsional vibration 4928.3.7 Variable speed stall regulation 4948.3.8 Control of variable slip turbines 4958.3.9 Individual pitch control 4968.3.10 Multivariable control – decoupling the wind turbine

control loops 4978.3.11 Two-axis decoupling for individual pitch control 4998.3.12 Load reduction with individual pitch control 5018.3.13 Individual pitch control implementation 5038.3.14 Further extensions to individual pitch control 5058.3.15 Commercial use of individual pitch control 5058.3.16 Feedforward control using lidars 505

8.4 Closed loop control: analytical design methods 5068.4.1 Classical design methods 5068.4.2 Gain scheduling for pitch controllers 5118.4.3 Adding more terms to the controller 5118.4.4 Other extensions to classical controllers 5128.4.5 Optimal feedback methods 5138.4.6 Pros and cons of model-based control methods 5168.4.7 Other methods 517

8.5 Pitch actuators (see also, Chapter 6 Section 6.7.2) 5188.6 Control system implementation 519

8.6.1 Discretisation 5208.6.2 Integrator desaturation 521References 522

9 Wind turbine installations and wind farms 5259.1 Project development 526

9.1.1 Initial site selection 5269.1.2 Project feasibility assessment 5289.1.3 The Measure-Correlate-Predict (MCP) technique 5299.1.4 Micrositing 5309.1.5 Site investigations 5309.1.6 Public consultation 5309.1.7 Preparation and submission of the planning application 531

9.2 Landscape and visual impact assessment 5339.2.1 Landscape character assessment 5349.2.2 Design and mitigation 5379.2.3 Assessment of impact 5389.2.4 Shadow flicker 5409.2.5 Sociological aspects 541

9.3 Noise 5429.3.1 Terminology and basic concepts 5429.3.2 Wind turbine noise 5469.3.3 Measurement, prediction and assessment of wind farm

noise 548


CONTENTS xv

9.4 Electromagnetic Interference 5519.4.1 Modelling and prediction of EMI from wind turbines 5539.4.2 Aviation radar 557

9.5 Ecological assessment 5589.5.1 Impact on birds 559References 562

10 Wind energy and the electric power system 56510.1 Introduction 565

10.1.1 The electric power system 56510.1.2 Electrical distribution networks 56610.1.3 Electrical generation and transmission systems 568

10.2 Wind farm power collection systems 56910.3 Earthing (grounding) of wind farms 57210.4 Lightning protection 57510.5 Connection of wind generation to distribution networks 57810.6 Power system studies 58110.7 Power quality 582

10.7.1 Voltage flicker 58610.7.2 Harmonics 58710.7.3 Measurement and assessment of power quality

characteristics of grid connected wind turbines 58910.8 Electrical protection 590

10.8.1 Wind farm and generator protection 59210.8.2 Islanding and self-excitation of induction generators 59410.8.3 Interface protection for wind turbines connected to

distribution networks 59610.9 Distributed generation and the Grid Codes 598

10.9.1 Grid Code – continuous operation 59910.9.2 Grid Code – voltage and power factor control 59910.9.3 Grid Code – frequency response 60110.9.4 Grid Code – fault ride through 60110.9.5 Synthetic inertia 602

10.10 Wind energy and the generation system 60210.10.1 Capacity credit 60310.10.2 Wind power forecasting 604References 607

Appendix A10 Simple calculations for the connection of wind turbines 609A10.1 The Per-unit system 609A10.2 Power flows, slow voltage variations and network losses 609

11 Offshore wind turbines and wind farms 61311.1 Development of offshore wind energy 61311.2 The offshore wind resource 616

11.2.1 The structure of winds offshore 61611.2.2 Site wind speed assessment 61611.2.3 Wakes and array losses in offshore wind farms 617


xvi CONTENTS

11.3 Design loads 62011.3.1 International Standards 62011.3.2 Wind conditions 62111.3.3 Marine conditions 62211.3.4 Wave spectra 62311.3.5 Ultimate loads: operational load cases and

accompanying wave climates 62411.3.6 Ultimate loads: non-operational load cases and

accompanying wave climates 63211.3.7 Fatigue loads 63411.3.8 Wave theories 63611.3.9 Wave loading on support structure 64411.3.10 Constrained waves 65711.3.11 Analysis of support structure loads 660

11.4 Machine size optimisation 66111.5 Reliability of offshore wind turbines 66311.6 Support structures 667

11.6.1 Monopiles 66711.6.2 Monopile fatigue analysis in the frequency domain 67411.6.3 Gravity bases 69011.6.4 Jacket structures 69511.6.5 Tripod structures 70211.6.6 Tripile structures 702

11.7 Environmental assessment of offshore wind farms 70411.8 Offshore power collection and transmission 707

11.8.1 Offshore wind farm transmission 70811.8.2 Submarine AC cable systems 71211.8.3 HVDC transmission 715

11.9 Operation and access 717References 719

Appendix A11 723References for table A11.1 723

Index 729


About the Authors

Tony Burton: After an early career in long-span bridge design and construction, Tony Burtonjoined the Wind Energy Group in 1982 to co-ordinate Phase IIB of the Offshore Wind EnergyAssessment for the UK Department of Energy. This was a collaborative project involvingBritish Aerospace, GEC and the CEGB, which had the task of producing an outline designand costing of a 100 m diameter wind turbine in a large offshore array. Following this, heworked on the design development for the UK prototype 3 MW turbine, before moving toOrkney to supervise its construction and commissioning. Later he moved to Wales to besite engineer for the construction and operation of Wind Energy Group’s first wind farm atCemmaes and he now works as a wind energy consultant.

Nick Jenkins was at the University of Manchester (UMIST) from 1992 to 2008. He thenmoved to Cardiff University where he is now Professor of Renewable Energy. His previouscareer had included 14 years industrial experience, of which five years were in developingcountries. He is a Fellow of the IET, IEEE and Royal Academy of Engineering and for threeyears was the Shimizu Visiting Professor at Stanford University.

David Sharpe has worked in the aircraft industry for the British Aircraft Corporation as astructural engineer. From 1969 to 1995 he was a Senior Lecturer in aeronautical engineeringat Kingston Polytechnic and at Queen Mary College, University of London. Between 1996and 2003 he was at Loughborough University as a Senior Research Fellow at the Centrefor Renewable Energy Systems Technology. David is a member of the Royal AeronauticalSociety and was a member of the British Wind Energy Association at its inception. He hasbeen active in wind turbine aerodynamics research since 1976.

Ervin Bossanyi: After graduating in theoretical physics and completing a PhD in energyeconomics at Cambridge University Ervin Bossanyi has been working in wind energy since1978. He was a research fellow at Reading University and then Rutherford Appleton Labo-ratory before moving into industry in 1986 where he worked on advanced control methodsfor the Wind Energy Group. Since 1994 he has been with international consultants GarradHassan where he is a principal engineer.


Preface to Second Edition

The second edition of the Wind Energy Handbook seeks to reflect the evolution of design rulesand the principal innovations in the technology that have taken place in the ten years since thefirst edition was published. A major new direction in wind energy development in this periodhas been the expansion offshore and so the opportunity has been taken to add a new chapteron offshore wind turbines and wind farms.

The offshore chapter begins with a survey of the present state of offshore wind farmdevelopment, before consideration of resource assessment and array losses. Then wave loadingon support structures is examined in depth, including a summary of the combinations ofwind and wave loading specified in the load cases of the IEC standard and descriptions ofapplicable wave theories. Linear (Airy) wave theory and Dean stream function theory areexplained, together with their translation into wave loadings by means of Morison’s equation.Diffraction and breaking wave theories are also covered.

Consideration of wave loading leads to a survey of the different types of support structuredeployed to date. Monopile, gravity bases, jacket structures, tripods and tripiles are describedin turn. In view of their popularity, monopiles are accorded the most space and, after anoutline of the key design considerations, monopile fatigue analysis in the frequency domainis explained.

Another major cost element offshore is the undersea cable system needed to transmitpower to land. This subject is considered in depth in the section on the power collection andtransmission cable network. Machine reliability is also of much greater importance offshore,so developments in turbine condition monitoring and other means of increasing reliability arediscussed. The chapter is completed by sections covering the assessment of environmentalimpacts, maintenance and access, and optimum machine size.

The existing chapters in the first edition have all been revised and brought up to date, withthe addition of new material in some areas. The main changes are as follows:

Chapter 1: Introduction This chapter has been brought up to date and expanded.

Chapter 2: The wind resource Descriptions of the high frequency asymptotic behaviourof turbulence spectra and the Mann turbulence model have been added.

Chapters 3 and 4: Aerodynamics of horizontal axis wind turbines The contents ofChapters 3 and 4 of the first edition have been rearranged so that the fundamentalsare covered in Chapter 3 and more advanced subjects are explored in Chapter 4. Somematerial on field-testing and performance measurement has been omitted to make


xx PREFACE TO SECOND EDITION

space for a survey of wind turbine aerofoils and new sections on dynamic stall andcomputational fluid dynamics.

Chapter 5: Design loads for horizontal axis wind turbines The description of IEC loadcases has been brought up to date and a new section on the extrapolation of extremeloads from simulations added. The size of the ‘example’ wind turbine has been doubledto 80 m, in order to be more representative of the current generation of turbines.

Chapter 6: Conceptual design of horizontal axis wind turbines The initial sections onchoice of machine size, rating and number of blades have been substantially revised,making use of the NREL cost model. Variable speed operation is considered in greaterdepth. The section on tower stiffness has been expanded to compare tower excitation atrotational frequency and blade passing frequency.

Chapter 7: Component design New rules for designing towers against buckling aredescribed and a section on foundation rotational stiffness has been added.

Chapter 8: The Controller Individual blade pitch control is examined in greater depth.

Chapter 9: Wind turbine installations and wind farms A survey of recent research onthe impact of turbines on birds has been added.

Chapter 10: Electrical systems New sections covering (a) Grid Code requirements forthe connection of large wind farms to transmission networks and (b) the impact of windfarms on generation systems have been added.


Acknowledgements forFirst Edition

A large number of individuals have assisted the authors in a variety of ways in the preparationof this work. In particular, however, we would like to thank David Infield for providing someof the content of Chapter 4, David Quarton for scrutinising and commenting on Chapter 5,Mark Hancock, Martin Ansell and Colin Anderson for supplying information and guidance onblade material properties reported in Chapter 7, and Ray Hicks for insights into gear design.Thanks are also due to Roger Haines and Steve Gilkes for illuminating discussions on yawdrive design and braking philosophy, respectively, and to James Shawler for assistance anddiscussions about Chapter 3.

We have made extensive use of ETSU and Risø publications and record our thanks tothese organisations for making documents available to us free of charge and sanctioning thereproduction of some of the material therein.

While acknowledging the help we have received from the organisations and individuals re-ferred to above, the responsibility for the work is ours alone, so corrections and/or constructivecriticisms would be welcome.

Extracts from British Standards reproduced with the permission of the British StandardsInstitution under licence number 2001/SK0281. Complete Standards are available from BSICustomer Services. (Tel +44 (0) 20 8996 9001).


Acknowledgements forSecond Edition

The second edition benefited greatly from the continuing help and support provided by manywho had assisted in the first edition. However, the authors are also grateful to the manyindividuals not involved in the first edition who provided advice and expertise for the second,especially in relation to the new offshore chapter. In particular the authors wish to acknowledgethe contribution of Rose King to the discussion of offshore electric systems, based on herPhD thesis, and of Tim Camp to the discussion of offshore support structure loading. Thanksare also due to Bieshoy Awad for the drawings of electrical generator systems, RebeccaBarthelmie and Wolfgang Schlez for advice on offshore wake effects, Joe Phillips for hiscontribution to the offshore wind resource, Sven Eric Thor for provision of insights andillustrations from the Lillgrund wind farm, Marc Seidel for information on jacket structures,Jan Wienke for discussion of breaking wave loads and Ben Hendricks for his input on turbinecosts in relation to size.

In addition, several individuals took on the onerous task of scrutinising sections of thedraft text. The authors are particularly grateful to Tim Camp for examining the sections ondesign loading, on- and offshore, Colin Morgan for providing useful comments on the sectionsdealing with support structures and Graeme McCann for vetting sections on the extrapolationof extreme loads from simulations and monopile fatigue analysis in the frequency domain.Nevertheless, responsibility for any errors remains with the authors. (In this connection, thanksare due to those who have pointed out errors in the first edition).

Tony Burton would also like to record his thanks to Martin Kuhn and Wim Bierbooms forproviding copies of their PhD theses – entitled respectively ‘Dynamics and design optimisationof offshore wind energy conversion systems’ and ‘Constrained stochastic simulation of windgusts for wind turbine design’ – both of which proved invaluable in the preparation ofthis work.


List of Symbols

Note: This list is not exhaustive, and omits many symbols that are unique to particularchapters.

a axial flow induction factor; flange projection beyond bolt centrea′ tangential flow induction factora′

t tangential flow induction factor at the blade tipa0 two-dimensional lift curve slope, (dC1/dα)a1 constant defining magnitude of structural dampingA, AD rotor swept areaA∞, Aw upstream and downstream stream-tube cross-sectional areasAc Charnock’s constantb face width of gear teeth; eccentricity of bolt to tower wall in bolted flange

jointbr unbiased estimator of βr

B Number of bladesc blade chord; Weibull scale parameter; dispersion of distribution; flat plate half

width; half of cylinder immersed widthc∗ half of cylinder immersed width at time t*c damping coefficient per unit lengthci generalised damping coefficient with respect to the i th modeC decay constant; wave celerity, L/T; constrained wave crest elevationC(v) Theodorsen’s function, where v is the reduced frequency: C(v) = F(v) +

iG(v)Cd sectional drag coefficientCD drag coefficient in Morison’s equationCDS steady flow drag coefficient in Morison’s equationCf sectional force coefficient (i.e. Cd or C1 as appropriate)C1, CL sectional lift coefficientCM inertia coefficient in Morison’s equationCm

n coefficient of a Kinner pressure distributionC p pressure coefficientCP power coefficientCQ torque coefficientCT thrust coefficient; total cost of wind turbine


xxvi LIST OF SYMBOLS

CTB total cost of baseline wind turbineCx coefficient of sectional blade element force normal to the rotor planeCy coefficient of sectional blade element force parallel to the rotor planeC(Δr, n) coherence – i.e. normalised cross spectrum – for wind speed fluctuations at

points separated by distance Δr measured in the across wind directionC jk(n) coherence – i.e. normalised cross spectrum – for longitudinal wind speed

fluctuations at points j and kd streamwise distance between vortex sheets in a wake; water depthd1 pitch diameter of pinion geardPL pitch diameter of planet gearD drag force; tower diameter; rotor diameter; flexural rigidity of plate; con-

strained wave trough elevationE energy capture, i.e. energy generated by turbine over defined time period;

modulus of elasticityE{} time averaged value of expression within bracketsE

[Hs

∣∣ U]

expected value of significant wave height conditional on a hub-height meanwind speed U

f tip loss factor; Coriolis parameter; wave frequency; source intensityf () probability density functionf1(t) support structure first mode hub displacementf j (t) blade tip displacement in j th modefin(t) blade tip displacement in i th mode at the end of the nth time stepf J (t) blade j first mode tip displacementf p wave frequency corresponding to peak spectral densityfT(t) hub displacement for tower first modeF force; force per unit lengthFX load in x (downwind) directionFY load in y directionFt force between gear teeth at right angles to the line joining the gear centresF(μ) function determining the radial distribution of induced velocity normal to the

plane of the rotorF() cumulative probability distribution functionF (x | Uk ) cumulative probability distribution function for variable x conditional on U =

Uk

g acceleration due to gravity; vortex sheet strength; peak factor, defined asthe number of standard deviations of a variable to be added to the mean toobtain the extreme value in a particular exposure period, for zero-up-crossingfrequency, v

g0 peak factor as above, but for zero upcrossing frequency n0

G geostrophic wind speed; shear modulus; gearbox ratioG( f ) transfer function divided by dynamic magnification ratioG(t) t second gust factorh height of atmospheric boundary layer; duration of time step; thickness of thin-

walled panel; maximum height of single gear tooth contact above critical rootsection

H hub height; wave height; hub height above mean sea level


LIST OF SYMBOLS xxvii

H1 1 year extreme wave heightH50 50 year extreme wave heightHjk elements of transformational matrix, H, used in wind simulationHi (n) complex frequency response function for the i th modeH ( f ) frequency dependant transfer functionHs significant wave heightHs1 1 year extreme significant wave height based on 3 hour reference periodHs50 50 year extreme significant wave height based on 3 hour reference periodHB breaking wave heightI turbulence intensity; second moment of area; moment of inertia; electrical

current (shown in bold when complex)IB blade inertia about rootI0 ambient turbulence intensityI15 expected value of hub height turbulence intensity at reference mean wind

speed of 15 m/sI+ added turbulence intensityI++ added turbulence intensity above hub heightIR inertia of rotor about horizontal axis in its planeIu longitudinal turbulence intensityIv lateral turbulence intensityIw vertical turbulence intensityIwake total wake turbulence intensityj

√−1k shape parameter for Weibull function; shape parameter for GEV distribution;

integer; reduced frequency, (ωc/2W ); wave number, 2π /L; surface roughnesski generalised stiffness with respect to the i th mode, defined as miω

2i

k1, k2 marine conditions reference period conversion factorsK constant on right hand side of Bernouilli equationKC Keulegan-Carpenter numberK P power coefficient based on tip speedKSMB size reduction factor accounting for the lack of correlation of wind fluctuations

over structural element or elementsKSx (n1) size reduction factor accounting for the lack of correlation of wind fluctuations

at resonant frequency over structural element or elementsKv () modified Bessel function of the second kind and order vK (χ ) function determining the induced velocity normal to the plane of a yawed

rotorL length scale for turbulence (subscripts and superscripts according to context);

lift force; wave lengthLx

u integral length scale for the along wind turbulence component, u, measuredin the longitudinal direction, x

m mass per unit length, integer; depth below seabed of effective monopole fixity;inverse slope of log-log plot of S-N curve

mi generalised mass with respect to the i th modemT1 generalised mass of tower, nacelle and rotor with respect to tower first modeM moment; integer; tower top massM mean bending moment


xxviii LIST OF SYMBOLS

M0 peak quasi-static mudline momentM1 (t) fluctuating cantilever root bending moment due to excitation of first modeMT teeter momentMX blade in-plane moment (i.e. moment causing bending in plane of rotation);

tower side-to-side momentMY blade out-of-plane moment (i.e. moment causing bending out of plane of

rotation); tower fore-aft momentMZ blade torsional moment; tower torsional momentMY S low-speed shaft moment about rotating axis perpendicular to axis of blade 1MZ S low-speed shaft moment about rotating axis parallel to axis of blade 1MYN moment exerted by low-speed shaft on nacelle about (horizontal) y-axisMZN moment exerted by low-speed shaft on nacelle about (vertical) z-axisn frequency (Hz); number of fatigue loading cycles; integer; distance measured

normal to a surfacen0 zero up-crossing frequency of quasistatic responsen1 frequency (Hz) of 1st mode of vibrationN number of blades; number of time steps per revolution; integer; design fatigue

life in number of cycles for a given constant stress rangeN (r ) centrifugal forceN (S) number of fatigue cycles to failure at stress level Sp static pressurep () probability density functionP aerodynamic power; electrical real (active) powerPm

n () associated Legrendre polynomial of the first kindq(r, t) fluctuating aerodynamic lift per unit lengthQ rotor torque; electrical reactive powerQa aerodynamic torqueQ rate of heat flowQ mean aerodynamic lift per unit lengthQD dynamic factor defined as ratio of extreme moment to gust quasistatic momentQg load torque at generatorQL loss torqueQm

n () associated Legrendre polynomial of the second kindQ1(t) generalised load, defined in relation to a cantilever blade by Equation (A5.13)r radius of blade element or point on blade; correlation coefficient between

power and wind speed; radius of tubular tower; radius of monopiler ′ radius of point on blader1, r2 radii of points on blade or bladesR blade tip radius; ratio of minimum to maximum stress in fatigue load cycle;

electrical resistanceRe Reynold’s numberRu(n) normalised power spectral density, n.Su(n)/σ 2

u , of longitudinal wind-speedfluctuations, u, at a fixed point

s distance inboard from the blade tip; distance along the blade chord fromthe leading edge; separation between two points; Laplace operator; slip ofinduction machine

s1 separation between two points measured in the along-wind direction

Wind Energy Jenkins Burton RED BOX RULES ARE FOR PROOF ... · The authoritative reference on wind...

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